# Question on power rating

1. Aug 10, 2015

### Lim Y K

For example
A computer with a power rating of 400w is switched on for 5 hours for a period of one week. What is the amount of electrical energy used?

I calculated using E=PxT
400w x (5hours x 7daysx 60mins x 60 seconds)
=50400000j

Why is this so? I thought its power (in watts) x time (in seconds)?

2. Aug 10, 2015

### Staff: Mentor

They are different units for energy. Look up the conversion factor between joules (which is what you have calculated) and kilowatt-hours (which is what you want).

Or, in your calculation of E = PT, use P in kilowatts and T in hours (instead of watts and seconds).

3. Aug 10, 2015

### Lim Y K

oh. but then shouldn't the claculation be 0.4kw x 60secomds x 60 mins? i thought power is the electrical energy used per second ? why is it 0.4w x 7 x 5?

4. Aug 10, 2015

### Lim Y K

or is it beacuse its kwh which means kw x h and not kw/h?

5. Aug 10, 2015

### A.T.

6. Aug 10, 2015

### CWatters

Correct.

I assume that is 5 hours per day for one week.

Energy (kWh) = Power (in KW) * Time (hours)

= 0.4 * 5 * 7 = 14 kWh

7. Aug 10, 2015

### Staff: Mentor

Power is energy used per unit time. The unit of time can be (is) different for different units. In this case, it is -- you just have to look carefully at the unit and use it as specified.